Traveling wave electron discharge devices



1957 J. H. BRYANT ET AL 2,798,981

TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Aug. 19, 1952 i Q l$ N) L L U5 INVENTORS ORN Y United States Patent TRAVELING WAVE ELECTRON DISCHARGE DEVICES John H. Bryant, Nutley, Herbert W. Cole, Cedar Grove, and Alexander W. McEwan, Pompton Lakes, N. 5., assignors to International Telephone and Telegraph Corporation, a corporation of Maryland Application August 19, 1952, Serial No. 305,153 16 Claims. (Cl. 315-35) This invention relates to traveling wave electron discharge devices and more particularly to a means of suppressing self oscillations in such devices.

The traveling wave type of tube is particularly useful in wideband microwave systems since it is capable of amplifying radio frequency energy over an unusually wide band of frequencies. The tube includes a form of transmission line, usually a helix, for transmission of microwave energy for interaction with an electron beam closely associated with the line. The helical characteristic of the transmission line is such that the axial velocity of microwave signals conducted along the helical path is approximately the same as or slightly slower than the velocity of the electrons of the beam, whereby the electrical field of the micro wave signals interacts with the electron beam for amplification of the microwave signals.

The copending application of J. H. Bryant and T. I. Marchese, Serial No. 221,862, filed April 19, 1951, entitled Traveling Wave Electron Discharge Device discloses a structural arrangement wherein a metallic envelope is employed rather than a glass or ceramic envelope. The structural arrangement therein described provides a more compact traveling wave tube wherein the terminals for the electrodes of the gun unit are located at one end of the tube, and the terminals for the radio frequency input and output lines are located at the other end of the tube.

Due to the structural arrangement of the metallic type of traveling Wave tube described in the above cited copending application or in a conventional glass or ceramic envelope tube contained in a closely-fitting metallic enclosure, there may be tendency for the tube to sustain self oscillations, particularly where there is any mismatch along the helical transmission line and when operated at high gain and high power levels. The construction thereof is such that the input R.-F. line, situated relatively close to the metallic envelope, forms in conjunction therewith a feedback transmission line. Any mismatch occurring on the radio frequency transmission line such as produced by minor imperfections in the line by the presence of imperfectly matched lossy material applied to a specific region of the line, or a mismatch occurring at the output R.-F. line, will produce energy fields which radiates into space, which in this case is into the surrounding structure of the input R.-F. line and metallic envelope. These radiating fields, therefore, tend to excite the transmission line formed by the input line and the metallic envelope. This radiated energy is thus fed back to the R.-F. input and thereby tends to produce self sustained oscillations.

It is therefore an object of this invention to provide stabilizing means to minimize the inherent tendency of the metallic type of travelingwave tube to oscillate.

A feature of this invention is to provide a plurality of circular baffies coaxial with the radio frequency transmission line and contiguous with the input R.-F. line having finger contacts associated with said bafiles and in contact with the inside surface of the metallic envelope, to attenuate any energy which tends to feedback along such structure.

Another feature of this invention is to provide a conducting cylinder disposed to join the input and output matching sections and coaxial with the radio frequency transmission line, having a diameter physically small as compared to a wavelength, for developing a high attenuation for electromagnetic radiation of energy to the aforeinentioned structurally formed feedback transmission Still another feature of this invention includes a plurality of suitable electrical shorting means disposed to join the outer conductor of the input R.-F. line and the metallic envelope, said shorting means being properly spaced closer together than one quarter wavelength, to effectively short out the aforementioned structurally formed, feedback transmission line and thereby remove the possibility of electromagnetic energy being propagated therealong.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 illustrates a view in elevation, partially in section, of an embodiment following the principles of this invention;

Fig. 2 illustrates a fragmentary view in elevation, partially in section, of another embodiment of this invention; and

Fig. 3 illustrates a fragmentary view in elevation, partially in section, of still another embodiment of this invention.

Referring to Fig. 1, the traveling wave tube of the metallic type is shown to comprise a cylindrical metallic housing 1 containing in one end thereof an electron gun unit 2 and in the opposite end thereof a radio frequency transmission line coupling unit 3 having associated therewith cooling fins 4 to dissipate the heat produced in the operation of this tube.

Referringto the radio frequency coupling unit 3, the unit includes an alignment plate and an annular diaphragm (not shown) cooperating in the purpose of mounting and positioning the input R.-F. coaxial waveguide 5 and the output R.-F. coaxial waveguide 6 along with an electron collector terminal (not shown) disposed axially within the radio frequency coupling unit 3. Associated with the inner side of the alignment plate is a radio frequency transmission line preferably in the form of a helix 7. If desired this transmission line may assume other configurations such as a plurality of annular disks or plates, whereby the axial velocity of the radio frequency energy is made preferably slightly slower than the velocity of the beam of electrons projected from the unit 2. The helical transmission line 7 may be supported by a quartz tubing 8, as shown, or line 7 may be supported by ceramic rods bonded to the helical conductor as described in the copending application of R. W. Wilmarth and B. D. McNary, Serial No. 305,228, filed October 19, 1952. Where the tubing 8 is employed to support the conductor 7, the tubing may include on its inner or outer surface or in the tubing material itself, for a portion of its length, lossy material 11, such as aquadag or molded graphite, to minimize reverse R.-F. conduction along the conductor 7 and the electric field thereof.

As hereinabove mentioned the metallic envelope of housing 1 and the input waveguide 5 structurally form a transmission line and as such may be excited to propagate electromagnetic energy from mismatches occurring at or near the output of helical conductor 7 to the input of the helical conductor 7 thereby tending to sustain self oscil- "ice lations.

coaxially of helical conductor 7 and contiguous with input waveguide 5. Bafiles 12, 13, and 14 are further provided with finger contacts 15 which electrically connect input waveguide to the metallic envelope of housing 1 through means of the baffles 12, 13, and 14. Therefore, the structurally formed feedback transmission line comprising metallic envelope of housing 1 and the input waveguide 5 is effectively shorted by means of the baffles 12, 13, and 14 in association with the finger contacts brazed or otherwise secured to baflles 12, 13, and 14 and making electrical contact with the metallic envelope. This effective short on the structurally formed transmission line presents a relatively large amount of attenuation to any electromagnetic energy which may be propagating in a feedback manner to the input of the helix 7 through the structurally formed transmission line and there-by We claim:

1. In a traveling wave electron discharge device having a metallic vacuum housing and a waveguide lengthwise of and within said housing which structurally forms with said housing a feedback transmission path which tends to be excited by radio frequency energy radiated within said housing and thereby produce self-sustained oscillations in said device; means to suppress conduction of radio frequency energy along said structurally formed transmission path comprising at least one conductive element disposed within said housing and in at least a 7 portion of said feedback path to short circuit radio freetfectively minimizes the inherent oscillations of the metallic type of traveling wave tube. Although three bafiles are herein shown, the actual number employed and the spacing thereof will depend upon the physical structure and frequency of operation for any particular application.

Referring to Fig. 2, another embodiment of means for suppressing oscillations in metallic type of traveling wave tube is illustrated. The metallic traveling wave tube illustrated herein is identical to that shown in Fig. 1 and the like reference characters are employed to indicate corresponding parts. A conducting cylinder 16 is disposed to join the input and output matching sections 9 and 10, respectively, thereby providing, if desired, a conductive cylinder extending the full length of the helical conductor 7. The diameter of cylinder 16 is necessarily small in physical dimensions as compared to a wavelength for reducing the possibility of reflecting radiated energy developed by a mismatch on the radio frequency transmission line in a manner to upset the normal interaction between microwave energy carried by the radio frequency transmission line and the electron beam. Therefore, the structurally formed transmission line comprising input waveguide 5 and the metallic envelope of housing 1 will not be excited. Any mismatch on the radio frequency transmission line 7 which produces fields that radiate will be intercepted by the cylinder 16 which acts as a shield.

Referring to Fig. 3, a further embodiment of suppression means for oscillations tending to occur in metallic traveling wave tube is illustrated. The means employed herein to minimize the feedback of radiated power comprises a plurality of shorting means 17 secured to input waveguide 5 and extending therefrom to make contact with the nearest inner surface of the metallic envelope of housing 1 and disposed with respect to each other less than one quarter wavelength. The shorting means 17 comprise spring contact 18 having secured thereto conducting bands 19 concentric to and in contact with wave guide 5. Rather than shorting means 17 being in two parts it is possible to produce such shorting means from one piece of stock.

Incorporating the shorting means 17, disposed from each other by less than one quarter wavelength, to reduce possibilities of reflection and resonance, the structurally formed transmission line is effectively short circuited and, therefore, will not propagate radiated energy in a feedback manner to the input of radio frequency transmission line 7, thereby minimizing the possibility'of self sustained oscillation.

The three embodiments of means to suppress self oscillations in metallic type of traveling wave tube illustrated herein are only representative of ways to accomplish the purpose of our invention. Other ways of accomplishing the purpose of our invention will be obvious to those skilled in the art and, therefore, the embodiments illustrated herein are not intended as limitations to the scope of our invention.

quency energy radiated within said housing.

2. In a traveling wave electron discharge device according to claim 1, wherein the suppression means includes a plurality of conductive bafiles disposed in spaced relation along said housing, each bafile being disposed crosswise of said housing.

3. In a traveling wave electron discharge device according to claim 1, wherein the suppression means includes a conductive cylinder disposed coaxially within said housing and electrically coupled to said housing to intercept energy radiated therein.

4. In a traveling wave electron discharge device according to claim 1, wherein the suppression means includes a plurality of conductive elements spaced along said waveguide to provide short circuit connections between the outer surface of said waveguide and said housing.

5. In a traveling wave electron discharge device according to claim 4, wherein said conductive elements each comprises a conducting band concentric with said waveguide and having spring contacts for contacting the inner surface of said housing.

6. A traveling wave electron discharge device comprising a metallic vacuum housing, an electron unit disposed at the forward end of said housing to project an electron beam along a given path in said housing toward the rearward end thereof, a radio frequency transmission means disposed adjacent to said path for propagation of radio frequency energy in interacting relation with the electrons of said beams, a radio frequency coupling unit disposed at the rearward end of said housing having an input line disposed lengthwise of and within said housing for coupling radio frequency energy to the forward end of said transmission means, said input line and said housing presenting structurally a transmission path, and means to suppress feedback conduction along said transmission path of any energy radiated from said transmission means, thereby minimizing any tendency of the device to sustain self oscillations.

7. A device according to claim 6, wherein the suppression means comprise a plurality of spaced elements of conducting material between said R.-F. line and said housing.

8. A device according to claim 6, wherein the suppression means comprise a conducting cylinder disposed coaxially of said transmission means between said input line and said transmission means to intercept energy radiated therefrom.

9. A device according to claim 8, wherein said conducting cylinder has a diameter physically small as compared to a wavelength of the R.-F. energy applied to said device.

10. A device according to claim 6, wherein the suppression means comprise a plurality of electrical shorting elements disposed to join said input R.-F. line and the inner surface of said housing to effectively short circuit said structurally formed transmission path.

11. A device according to claim 10, wherein said shortening elements each comprises a conducting band concentric with said input R.-F. line and having spring contacts for contacting said inner surface of said housing.

12. A device according to claim 6, wherein the suppression means comprise bafiles having circumferentially located finger contacts, said battles being disposed coaxially of said transmission means and contiguous with said input R.-F. line to present high attenuation to energy propagating along said transmission path.

13. A device according to claim 6, wherein the suppression means comprises a conducting cylinder disposed coaxially of said transmission means for the entire length thereof to intercept any energy tending to radiate from said transmission means.

14. A device according to claim 6, wherein the suppression means comprises a plurality of electrical shorting elements disposed to join said input R.-F. line and the inner surface of said housing to efiectively short circuit said structurally formed transmission path, said shorting elements being separated from each other by less than one quarter wavelength.

15. In a traveling wave electron discharge device according to claim 2, wherein said baffles are spaced from each other by less than one quarter of the operating wavelength.

16. In a traveling wave electron discharge device according to claim 4, wherein said conductive elements are spaced from each other by less than one quarter of the operating wavelength.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,647 Lindenblad Apr. 21, 1953 1,957,538 Jensen May 8, 1934 2,144,222 Hollmann Jan. 17, 1939 2,157,952 Dallenbach May 9, 1939 2,602,148 Pierce July 1, 1952 2,608,668 Hines Aug. 26, 1952 2,636,948 Pierce Apr. 28, 1953 2,672,571 Harman Mar. 16, 1954 2,701,322 Huber Feb. 1, 1955 

